The present application claims priority from European Patent Application No. 01660154.4, filed Aug. 30, 2001.
The invention relates to a geometrical beam splitter for transversally dividing a radiation beam into at least one reflected beam portion and at least one passing beam portion, said beam splitter being composed of a piece of rigid material having a non-transparent reflective surface at an angle in respect to the incident direction of said radiation beam, said angle substantially deviating from the right angle. The invention also relates to a sensor comprising a radiation source, a measuring chamber, at least two detectors, at least two optical filters each of which between the radiation source and one of the detectors, and a beam splitter composed of a piece of rigid material and being at least partly reflective; whereupon a radiation beam from said radiation source travels to said at least two detectors through the measuring chamber and through the respective optical filters, said beam splitter positioned between the detectors and the measuring chamber so as to allow a reflected portion and an undiverted portion of said radiation beam to reach the detectors simultaneously.
Beam splitters are used in optics for the purpose of combining two beams, and for separating one beam into two. Wavelength region or distribution and intensity ratio between the two separated beam portions depends upon the specific properties of the beam splitter. The most typical beam splitter is a thin plate of glass or plastic with one surface coated with a semi-reflecting coating or semi-transparent mirror coating. One portion of the beam is transmitted through the beam splitter and the other portion is reflected typically by 90 degrees. Possible absorption in the beam splitter materials is here ignored. The drawbacks caused by the reflection from the second glass surface can be avoided by using a beam splitter cube. It consists of two right angle prisms cemented together. The hypotenuse of one prism is coated with a semi-reflecting coating before cementing. The construction is expensive, especially if the wavelengths in use are in the infrared region with few suitable materials. Other type of prisms and combination of prisms are also known. Further a thin semi-reflecting membrane, a pellicle, is a possible solution but it may not be robust enough in many cases and it can be sensitive to temperature fluctuations, and its reliable fastening is also a problem. The beam splitters described above are called physical beam splitters because the complete beam aperture is available in both the transmitted and the reflected part. Physical beam splitters are described e.g. in publication Naumann/Schrxc3x6der: BAUELEMENTE DER OPTIK Taschenbuch der Technische Optik; Carl Hanser Verlag 1987, pp. 186-187, and the use of a beam splitter can be found in the publication U.S. Pat. No. 5,908,789.
Another alternative of the beam splitters are so called geometrical beam splitters, in which the beam cross-section is either divided into two portions having different wavelength distribution by using a grating or metallic grid or a mesh, or divided into two portions with the same wavelength distribution both having a smaller cross-section area than the initial beam by using reflecting stripes or spots e.g. on a glass plate or a prism or by using a mirror to cover a section of the initial beam. The latter type of beam splitters are often used in the infrared region, but avoiding radiation absorption of the material requires use of special materials, which may cause problems in some applications, because the material has to be thin and a robust support with little temperature dependence is also in this case very difficult to construct. The gratings, grids and meshes are described in publication W. Driscoll, W. Vaugham: HANDBOOK OF OPTICS, McGraw-Hill Book Company 1978, pp. 8-106-8-109 do not suffer radiation absorption problems, but the feature that the transmitting portion and the diverted portion has different wavelength distributions is not acceptable for many purposes. The geometrical beam splitters for cross-sectional dividing are disclosed in publications Naumann/Schrxc3x6der: BAUELEMENTE DER OPTIK Taschenbuch der Technische Optik; Carl Hanser Verlag 1987, pp. 186-187, and Module 6xe2x80x946 xe2x80x9cFILTERS AND BEAM SPLITTERSxe2x80x9d, Center of Occupational Research and Development, 1987 {http://www.cord.org/cm/leot/course06}. FIG. 29 in the last mentioned publication shows a planar mirror with an aperture, the mirror being perpendicular to the radiation direction. This kind of mirror construction is used solely in high power CO2-lasers, in which semitransparent mirrors cannot be used because of the extremely high power of several kW""s requiring cooling. In these CO2-lasers, utilized for welding and cutting metals, said mirror with aperture is used as one of the end mirrors, whereupon the main portion of the light is reflected directly back to the other mirror at the opposite end of the laser, and the productive laser power beam comes out through the aperture. FIG. 28 in the last mentioned publication shows a plant mirror partly protruding in the incident light beam and so dividing it into one smaller portion of reflected light and one larger portion of undiverted light. This alternative has the drawback not being robust or steady and it is also difficult to manufacture in small sizes with a precision high enough especially for modern sensors with several detectors.
Publications JP-05-215 683 discloses a device for analyzing e.g. the concentrations of gas components in a gas mixture on the basis of the absorption of infrared radiation. The device comprises a radiation source, the radiation emitted thereby being aligned to travel through a measuring cell, which contains the gas mixture to be analyzed, a first optical filter, which is positioned on the path of radiation, and a first detector, positioned in the radiating direction downstream of said first filter and used for detecting the radiation intensity falling thereon. The device further includes at least a second optical filter provided with a detector for identifying and/or measuring the concentration of at least one other gas component. In order that these at least two separate detectors simultaneously receive radiation from the measuring cell, the device is further provided with a beam splitter. According to the publication the beam splitter can be of the type of the semi-reflecting coating or semi-transparent mirror coating, as described above. Alternatively this publication suggests using a reflecting mirror, in the center of which an aperture is punched for passing a portion of the incoming radiation and followed by a gas filter and a detector. Also JP-05-215684 discloses a gas analyzer with a plurality of detectors. However, the beam splitter is composed only of reflecting parts. No transmitted portion of the beam is shown or described.
The publication U.S. Pat. No. 6,122,106 describes an opto-mechanical system to be used as a laser transmitter/receiver for measuring distances. The incoming light is actually not divided into a reflected and a passing beam portion but there is only a reflected portion. The two holes in the mirror are used for transmitting radiation in the opposite direction as compared to the incoming and reflected light. According to the publication these two holes are as small as possible, like xe2x80x9cpencil thinxe2x80x9d, so that the reflected portion is maximized, whereupon the area of the radiation transmitting in inverse direction is extremely small as compared to the area of the reflected radiation. The publication JP-63-107082 describes a laser mirror. It has one or a plurality of very small holes like pinholes in it, whereupon the reflective area is many orders larger than the area of the small holes. The laser light transmitted through this/these small hole(s) forms accordingly an extremely small portion of the whole radiation, which indeed is enough in this case, because the transmitted portion is directed to one detector, which is used for controlling the laser oscillator only. Copper and molybdenum is suggested as the body material and gold is used as a reflective coating.
Publication U.S. Pat. No. 4,940,309 describes a device that divides or brakes down an incoming wavefront into several non-overlapping portions, i.e. into image sub-sections, and the device is a scanner called xe2x80x9ctesselatorxe2x80x9d The idea is to make a large imaging surface using a plurality of small imaging surfaces. The publication suggest using one or several glass plate(s) with reflective areas of back-coated mirror, the plate(s) protruding into the area of the whole wavefront. It is so preferred that the material of the dividing components is transparent to radiation, and this transparent material has areas of metallic coating, which structure promotes avoiding distortion. Accordingly each detector is here arranged to receive radiation from separate parts of the object.
U.S. Pat. No. 1,253,138 and EP-0 635 745 disclose in principle similar light splitting devices, the U.S.-patent for color photography and the EP-patent application for measuring purposes in ultraviolet region. Both publications suggest using a mirror as thin as possible, whereupon EP-publication defines that the material of the mirror is foil having thickness less than 0.0762 mm, and the mirror has preferably a plurality of holes. Since the beam splitter of the U.S.-patent is part of an imaging device, it is especially important that the holes in the mirror are small and numerous, as disclosed in the publication. Both publications suggest that the walls of the holes are inclined or overcut in order to avoid reflections or scattering from them, and U.S.-patent further says that the walls are made xe2x80x9cdead blackxe2x80x9d. This is understandable since such reflections would e.g. blur the image on the photographic plate. These are typical geometrical beam splitters. The wavefront is divided into two more or less identical portions by the many holes in the beam splitter. The detector or film for the transmitted portion and the detector or film for the reflected portion get information from the whole wavefront, which means that each point of e.g. a photographic plate receives radiation from every hole present in the perforated mirror. Both of these publications disclose one single detector/film for the transmitted radiation and one single detector/film for the reflected portion.
In non-dispersive multi-gas detection several detector elements with respective optical filters having narrow pass-bands of wavelengths are used to identify and measure the concentration of the different infrared absorbing gas components. When using discrete detector packages a robust construction is possible but the size of the sensor is a limiting factor with today""s demand for compact measuring devices. One possibility is to integrate all detector elements and optical filters into one package. It is possible to install even more than five detector elements within such a package. However, in order to have all detector elements directed approximately at the same part of the gas sample, the individual elements and optical filters have to be very small. This means reduced signal and less measurement reliability. Potentially lower yield may even suggest that the detector package is expensive to manufacture because of the many small optical filters and crosstalk suppression constructions. A better solution would be to mount the detector elements in two packages and to use a known physical beam splitter, i.e. a semitransparent mirror. Then the detector elements and filters can be larger, because the same package cross-section has fewer elements. The drawback in this case is the intensity reduction introduced by the conventional beam splitter with the typical splitting ratio 50%-50%, meaning that the intensity of the transmitted beam is equal to that of the reflected beam. This is valid independently of how the detector package is positioned in respect to the optical axis.
The main object of the present invention is to overcome the described drawbacks and to provide an inexpensive and robust beam splitter to be used with at least two detector packages in different positions. Preferably at least one of the detector packages is of multi-element type. The second object of the present invention is to provide a small sized or miniature sized beam splitter. The third object of the invention is to provide a beam splitter, which allows directing the detectors to measure the same area or volume in a measuring chamber. The fourth object of the invention is to provide a beam splitter, which is without problems applicable to be used with infrared radiation, too. The fifth object of the invention is to provide a beam splitter which divides the incoming radiation beam into portions having at least near identical wavelength range and at least near identical radiation intensity going to the detector packages or detectors prior to optical filters. The sixth object of the invention is to provide a beam splitter, which delivers radiation to each of the detectors with a minimum of losses, i.e. with a high efficiency. Further it is an object of the invention to provide a sensor with a beam splitter, which would be capable for analyzing with high accuracy and reliability several gas components in a gas mixture, without movable parts and with properties as set forth above.
The above-mentioned problems can be solved and the above-defined object can be generally achieved by means of a geometrical beam splitter according to the invention as defined in claim 1, especially in analyzing gas components in a gas mixture by means of a sensor with a geometrical beam splitter according to the invention as defined in claim 16. The principal idea for attaining this kind of a beam splitter is to construct the beam splitter surface with one hole for each detector element of the first package in the transmitted beam and by adjusting the second detector package within the optical confinement of the larger package to receive the reflected beam. By doing this it is possible to increase the computational beam splitting ratio even beyond 90%-90%. The beam splitter gain, i.e. the radiation directed to the detectors, is so approaching 100% as compared to the intensity of the initial radiation coming from the measuring chamber or any other source. This seemingly impossible ratio means that the beam splitter does not necessarily introduce any notable intensity losses. Ideally, these two portions are more or less identical regarding spatial distribution. In this invention the beam splitter is achromatic, which means that it is essentially independent of the wavelength region in use and that there is minimal influence on the beam polarization.
The hole or holes in the geometrical beam splitter according to the invention is/are typically of the same size as or slightly larger than the individual detector elements to allow collimated or nearly collimated radiation to pass through into one or several first detector elements e.g. in the first package. The areas that reflect collimated or nearly collimated radiation to one or several second detector elements e.g. in the second package are proximate to the holes. The total area of the geometrical beam splitter according to inventions preferably extends over the cross-section of the initial radiation beam coming into this beam splitter, in which case the beam splitter does not introduce any losses if good reflectivity is assumed. The hole or holes and the reflective area or areas are close to each other so that each of the detectors receive radiation substantially from the same area or volume of the measuring chamber or from the same area or volume of some other source of interest. Because of the few large holes the beam splitter can be much thicker than what is possible in the prior art constructions. This also means that the plate is robust and easy to mount. Thermal bending can be avoided by using a thermally similar material or even the same material as the rest of the sensor body, preferably aluminum. Such a plate is also easy to polish and mount. A further advantage is that the hole(s) can be formed to function as wavetube(s), as described in patent U.S. Pat. No. 5,610,400 of the applicant, because of the thickness and the metallic material of the beam splitter, which further increases the effectiveness of passing radiation. Though the individual reflecting area(s) are substantially of the same size as the cross-section(s) of the hole(s) is the total reflective areaxe2x80x94total area minus area of the hole(s)xe2x80x94large contributing an increased effectiveness of reflected radiation. The detector elements are normally fitted with individual optical filters with narrow pass-bands so that each detector element detects radiation within a different wavelength region. These regions are determined by the spectral absorption peaks of the specific gases to be measured. It shall be understood that though the geometrical beam splitter according to the invention is ideal in the sensor for the multi-gas analyzing, it can be a most useful beam splitter for other type of technical applications, too.